Though it's rare, mares can occasionally demonstrate behavior that people commonly associate with stallions — acting aggressively, dominating herdmates and even teasing or mounting other mares. Dr. Patrick McCue tells the American Quarter Horse Association that there are several possible causes for these behaviors.
These include:
Ovarian tumors – The most common ovarian tumors are granulosa cell tumors. About 40 to 50 percent of granulosa tumors contain theca cells, which produce large amounts of testosterone and can cause a mare to act like a stallion. Testosterone levels will remain high until the mare has the tumor surgically removed.
Pregnancy – Pregnant mares have elevated testosterone levels while their fetus produces gonads; the testosterone crosses the placenta into the mare's bloodstream and can affect her behavior. Mares begin to have increased testosterone levels around their fourth month of gestation and these levels peak around month seven. By the last trimester of pregnancy, as testosterone levels decrease, mares tend to become less aggressive.
Mares that aren't pregnant can experience testosterone fluctuations during their estrous cycles.
Steroids – Anabolic steroids can suppress estrus and increase stallion-like behavior, including aggression. These behaviors resolve once steroid use is terminated.
Pseudohermaphrodism – Though rare, pseudohermaphrodism in horses occurs when a horse looks like a mare, but has the genetic makeup of a male horse. These horses have testes instead of ovaries; they are cryptorchids that don't produce spermatozoa, but the testes produce enough testosterone to affect the horse. Horses that are pseudohermaphrodites may have female genitalia. Removing the testes will eliminate stallion-like behavior.
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